Role of helper T cells in B cell Response: Comparison of naive and memory B cells. Long-lived protective immunity to pathogens depends on the generation, during the first exposure, of memory lymphocytes which are specialized to respond quickly and effectively to re-encounter with the same pathogen. Naive B cells specific for any particular pathogen are present at low frequencies and their activation and proliferation are essential for effective primary Antibody (Ab) response. After immunization, memory B cells are present at increased frequencies, they have often switched from production of IgM Ab to production of more effective isotypes such as IgG and the Ab they now make is of much higher affinity for the antigens seen during the primary response. Other features of naive and memory B cells are much less clear. A major goal in this proposal is to devise methods to identify and isolate naive and memory B cells, using mice transgenic for the B cell Ab Receptor. We will then determine the properties of naive and memory B, including their lifespans, in vivo turnover and capacity to home to key lymphoid tissues. We will investigate in detail the requirements of naive and memory B cells for T cell help, including the role of the direct recognition of the B cell by the T cell, the role of interactions between so-called "costimulatory" molecules on the two cells and the role of cytokines produced by helper T cells. Different T cell subsets will be compared for their ability to help, including newly defined subsets of CD8 T cells making distinct patterns of cytokines. We will also examine whether the role of the helpers and their products includes rescuing the B cells from programmed cell death. In later stages of the grant, we will use the information obtained above to examine the role of cytokines and T cell subsets in the in vivo primary response and in generation of B cell memory and we will develop in vitro models to study the generation of B cell memory and the factors which regulate that process. These studies should provide important new insights into the most effective procedures for generating primary responses to new antigens, for generating protective immunity and for eliciting an optimum response after priming.